In Chapter 4, after the record breaking trip, Francois and Perrault are transferred to some other duty and Buck and his pack head back to Dawson.  I think this is the trip that you are talking about.

In this trip, things are much harder than in the previous one.  This time, the dogs are already tired and the mail is very heavy.  The dogs should have been given time to rest, but they weren't.

What made things worse was the snow.  It snowed every day on the trip and this meant that the trail was soft.  A soft trail meant more friction and that meant it was harder for the dogs to pull the sled.